The present invention relates to a document data creating apparatus for creating document data, a document data creating method to be employed in the abovementioned apparatus and a control program to be executed in abovementioned apparatus concerned.
In recent years, there have been increasingly proliferated in the market various kinds of copiers or multi-functional apparatuses, each provided with a combination of plural functions, such as a copy function, a facsimile function, a printer function, a scanner function, etc., (hereinafter, referred to as an image forming apparatus as a general term for each of them). When the image forming apparatus is employed as the network printer to implement the printing operation, at first, the concerned document is created by using the application program installed in advance in the computer terminal device coupled to the network, and then, the created document is converted to the document data in the format of the Page Description Language (hereinafter, also referred to as the PDL, for simplicity), so as to be transmitted to the image forming apparatus concerned. Receiving the document data from the computer terminal device, the image forming apparatus parses the document data to create intermediate data, and then, further converts the intermediate data to bitmap data, so as to output an image based on the bitmap data onto a paper medium.
On that occasion, since there is a fear that the print data to be transmitted through the network might be possibly falsified during the transmitting operation of the document data, the method for judging the validity of the document data by employing a digital signature has been usually employed. For instance, Japanese Non-Examined Patent Publication No. 2005-267022 sets forth a method in which: the digital signature and the electronic certificate attached to the document data concerned are used to decrypt the digital signature by employing the public key included in the electronic certificate to acquire hash values; the hash values inherent to the document data are compared with the hash values acquired; the document data is determined to not to be falsified if the hash values inherent to the document data coincide with those acquired; document data including hidden character information is created to output to the printer concerned; and thus the printer prints an image represented by the document data including the hidden character information.
According to the conventional method mentioned in the above, there has been such a problem that, although it is possible to detects the manipulation (falsification) made by third person after the document data have been created, it is unable to effectively utilize the digital signature, since none of countermeasures are taken into account to prevent manipulations and modifications, such as, an artificial change made by the operator, an unintentional change caused by an apparatus characteristic, etc., which would be possible made during the creating operation of the document data.